Integrate WiredTiger verbose messages with MongoDB logging

WiredTiger debug messages (i.e., output from verbose messaging) should be integrated with MongoDB so a user can ask for more detailed or component-specific messages from WT just as they can from other parts of MongoDB, and the resulting messages will be captured and written to the mongod logpath.

Messages from WT already get captured and written to the MongoDB log. This ticket addresses the other parts that we need for integrated with mongod:

1. Provide a WT API that would allow the user (i.e., mongod) to set a severity level and specify one or more components to enable additional messaging from.
2. Add new messages as appropriate to provide the data requested via this API.  

The existing verbose=[...] WT config option provides component-level logging from WT.  So one way to approach this would be to add a severity option and assign a default severity level to all the existing verbose messages.  Then we can adjust severities and add/remove messages incrementally as the need arises or with directed tickets to revise the debuting messages in specific areas of the system.